The double-lock-stitch gripper which is predominantly used in this case is a rotating gripper that executes two complete rotations per stitch-forming period, as is employed hundreds of thousands of times as a “standard looper” in sewing machines and has proved very successful here too. This double-lock-stitch gripper requires a particular amount of thread tension for stitch forming, in particular for stitch insertion.
The use of a double lock stitch looper performing two full rotations per stitch-formation cycle and thus rotating two times per stitch formation is known in sewing machines, for example, through DE 42 17 848 C1.
The sewing machine disclosed here has, as is well known, a base plate, a column and an arm, which ends in a head. In the arm is mounted an arm shaft, which is in drive connection with a needle bar carrying a thread-guiding needle that is movable up and down in the head.
As is well known, a thread lever, which cooperates with the needle, which performs an up and down movement and which is likewise driven by the arm shaft, is also mounted in the head. However, the arm shaft is in drive connection via a belt drive or via a mechanical drive with a looper drive shaft mounted in the base plate of the sewing machine and drives same at a ratio of 1:2, so that the looper performs two rotations per each stitch-formation cycle, whereby the respective first rotation is used for detecting and expanding the needle thread loop, while the respective second rotation represents an “empty rotation,” which has no effect on the stitch formation.
The different purposes of a sewn seam on the one hand and an embroidered seam on the other are the basis for a substantial difference between the requirements of a sewing machine and those of an embroidery machine. Whereas a sewn seam represents essentially a connecting or fastening seam, with which the usually two or more parts are connected firmly together, an embroidered seam represents a decoration (decorative seam) applied to the embroidery cloth, and transmits no forces at all.
The strength of the fastening seam is achieved in this case in that the thread loop dropped by the gripper is pulled by the thread lever with a relatively great force towards the top side of the article to be sewn so that the knotting of the needle thread and the gripper thread comes to lie centrally in the layers of material to be sewn together. The thread force of the gripper thread should also be calculated accordingly.
The conditions in the formation of an embroidered seam are completely different. Since in this case the needle thread (=embroidery thread) is only intended to lie cleanly on the top side of the embroidery material in order to obtain decorative effects, during the embroidery process the thread tension is kept as low as possible such that, while the embroidery thread is secured sufficiently in position on the embroidery cloth, no loose thread components arise and the knotting between the needle thread and the gripper thread always comes to lie on the underside of the embroidery cloth.
For this reason, a much lower tension of the needle thread is desired for embroidery machines compared with sewing machines.
Since, when the double-lock-stitch gripper that executes two rotations per stitch-forming period was transferred into the embroidery machine, the kinematics of said double-lock-stitch gripper were not adapted in an appropriate manner, even in current embroidery machines the feed movement for the embroidery cloth commences at a point in time at which the stitch-forming process has not yet been completed. This is very disadvantageous because as a result, when the thread lever starts to pull in the thread, the embroidery cloth has already moved by a considerable amount with respect to its position when the needle is inserted into the embroidery cloth. Consequently, the limb of the thread loop leading from the stitch hole to the thread store experiences in each case an additional deflection both when it emerges from the stitch hole and immediately after it passes through the embroidery cloth. The two deflections cause additional resistance when the thread is pulled in by the thread lever, and this additional resistance inevitably leads to an undesired increase in the minimum amount of thread tension.
This situation has a particularly disadvantageous effect in embroidery machines overall because, compared with sewing machines, they generally operate both with much greater stitch lengths and also with feed directions that extend in different directions.